In theory it was espionage or sabotage, in practice it was mostly ethnicity, i.e. person with wrong background at a wrong time in the wrong place. The bad luck of being in sight when someone had to meet his efficiency target in the process of purging any political opposition.

In this particular case, ethnic Ingrian near Leningrad. (Ingrians were the Finnish-speaking population in Leningrad oblast, i.e. people who had stayed there since Sweden ceded the area to Russia in the Treaty of Nystad ending the Great Northern War in 1721, and Peter the Great started to build St. Petersburg.)

Speaks a language other than Russian, confesses a minority religion, could quite easily be a spy and get information and then slip across the border. No real evidence was needed to put people in camps, kill them, or declare them enemies of the people. Stalin's repression killed hundreds of thousands in extermination camps where, by the way, Nazi officials visited to learn the trade of killing by hunger.

Today, the ethnic cleansing of Ingrians is all but complete; it was finalized by Finland allowing the Finnish-speaking population to immigrate in 1990s, leaving only some old people who are all soon dead.

To clarify, of course also ordinary Russians were thrown into this machinery of destruction. Another ethnicity just increased the likelihood quite a lot. Stalin himself was Georgian, not Russian.

The joking advice given to people eaten up by this machinery of killing was:

  1. Do not confess anything.
  2. If you confess, do not sign anything.
  3. If you confess and sign, do not be surprised.

Of course, in reality, upon entering the system you were most likely dead already.